1. Field of the Invention
The present invention relates to a method and to an apparatus for coding pulse-amplitude modulated signals into pulse-code modulated signals in accordance with an approximated logarithmic coding characteristic curve, whose dynamic region is sub-divided into a plurality of segments with a coding which is linear in itself. The method includes a pre-decision operation which is dependent upon the amplitude of the input signal and which subjects below or above a specified threshold to an amplitude transformation. The process converts all input values, which have been compressed in such a manner, in accordance with a partial coding characteristic curve obtained from the coding characteristic curve by the same amplitude transformation.
2. Description of the Prior Art
In order to reduce the data rate, it is known in the conversion of pulse-amplitude modulated signals to carry out a compression by an approximated logarithmic characteristic curve. In practice, a so-called 13-segment coding characteristic curve (CCITT) has prevailed on the market. In this characteristic curve, the entire region for positive and negative amplitudes is divided into 13 linear segments. With these sections, which have different inclinations, the amplitudes are again linearly coded. The German allowed and published application No. 17 62 846 fully incorporated herein by this reference, discloses a process and a device for coding pulse-amplitude modulated signals into pulse code modulated (PCM) signals. In this process, the input signal is sub-divided into two or more amplitude segments. By multiplication of the small input signals, identical coding characteristic curves are produced over at least one segment, so that the dynamic scope is limited and a simpler coding arrangement results. As the coding device is often used in multiplex operation, high requirements are placed on the speed of processing.